Acaricide, a chemical used against Varroa mites that infect
honeybees, appears to render bees more susceptible to deformed wing virus
infections, according to research published in the January issue of the journal
Applied and Environmental Microbiology.
Like the mites, these viruses have been identified as potential causes of
colony collapse disorder.

The Varroa mite is currently the main pathogen linked to colony
collapse disorder among European honey bees worldwide. “The mite population
grows rapidly, killing colonies within 2-3 years if beekeepers don’t remove the
mites, which usually involves chemical treatments such as acaricides,” says
first author Barbara Locke, of the Swedish University of Agricultural Sciences,
Uppsala. Nonetheless, she says, the mites kill the colonies not by direct
effects, but by transmitting viral infections to the bees.

Deformed wing virus is strongly associated with mite infestations,
and the researchers’ hypothesized that following acaricide treatment, the virus
population would drop along with the mite population. In the study, they treated six bee colonies with Apistan, an
acardicide, for six weeks (the standard treatment duration) and left three
control colonies untreated, monitoring mite infestation and virus levels
weekly.

Contrary to the investigators’ hypothesis, the viral
infection worsened in the treatment group immediately following acaricide
treatment. “This initial increase was seen in all bee stages, including pupae
that never were in contact with mites,” says Locke. “Thus, we interpreted it to
be a possible direct effect of the acaricide, making the bees more susceptible
to virus infection,” possibly due to either “debilitating direct effects of
tau-fluvalinate on honeybee physiology and/or immune system responses,” she
says, adding that further studies are needed to confirm this result. The virus
infections ultimately dropped, slowly, “due to reduced transmission by the
mites,” says Locke. “However, even at the end of the mite removal treatment, we
still recorded substantial infection levels.”

Two other viral infections, black queen cell virus, and sac
brood virus, were found not to be associated with the mite infestation, says
Locke.

“Acaricide treatments are still the most effective method at
removing mites from colonies when infestation is high, at colony mortality
thresholds,” says Locke. “However, this research suggests that for maintaining
low infestation, regular alternative control treatments, such as organic
methods, could be used for promoting better bee health overall. Further, since
the virus is not reduced in the colony as rapidly as the mites are removed, our
research underlines the importance of early mite removal to reduce virus levels
in overwintering bees, to avoid colony losses.”